Cooperative Chloride Hydrogel Electrolytes Enabling Ultralow-Temperature Aqueous Zinc Ion Batteries by the Hofmeister Effect

Anionic Hofmeister Effect Regulated Conductivity in Polyelectrolyte Hydrogels for High-Performance Supercapacitor

The Hofmeister effect not only affects the stability and solubility of protein colloids but also has specific effects on the polymer molecules. Here, the impact of the Hofmeister effect on the electrochemical properties of polyelectrolyte hydrogels at room temperature and subzero temperature studied for the first time. Polyelectrolyte hydrogels exhibit an anti-polyelectrolyte effect in low concentrations of ammonium salt, while they exhibit an obvious Hofmeister effect in high concentrations of ammonium salt. Kosmotropic ions demonstrate strong interaction with water molecules or polymer chains, resulting in the reduction of conductivity of polyelectrolyte hydrogels. However, chaotropic ions exhibit weak interactions with water molecules or molecular chains, leading to an increase in conductivity. The Hofmeister effect has a more significant effect on the polyzwitterion electrolyte. The conductivity of polyzwitterion hydrogel soaked in chaotropic ion is up to 6.2 mS cm-1 at -40 °C. The supercapacitor assembled by polyzwitterion electrolytes maintains a capacitance retention rate of 85% and ≈100% coulomb efficiency after 15 000 cycles at -40 °C. This study elucidates the influence of the Hofmeister effect on conductivity in polyelectrolytes and expands the regulatory approach for improving the performance of energy storage devices.

Lamellar Nanoporous Metal/Intermetallic Compound Heterostructure Regulating Dendrite-Free Zinc Electrodeposition for Wide-Temperature Aqueous Zinc-Ion Battery

Aqueous zinc-ion batteries are attractive post-lithium battery technologies for grid-scale energy storage because of their inherent safety, low cost and high theoretical capacity. However, their practical implementation in wide-temperature surroundings persistently confronts irregular zinc electrodeposits and parasitic side reactions on metal anode, which leads to poor rechargeability, low Coulombic efficiency and short lifespan. Here, this work reports lamellar nanoporous Cu/Al2Cu heterostructure electrode as a promising anode host material to regulate high-efficiency and dendrite-free zinc electrodeposition and stripping for wide-temperatures aqueous zinc-ion batteries. In this unique electrode, the interconnective Cu/Al2Cu heterostructure ligaments not only facilitate fast electron transfer but work as highly zincophilic sites for zinc nucleation and deposition by virtue of local galvanic couples while the interpenetrative lamellar channels serving as mass transport pathways. As a result, it exhibits exceptional zinc plating/stripping behaviors in aqueous hybrid electrolyte of diethylene glycol dimethyl ether and zinc trifluoromethanesulfonate at wide temperatures ranging from 25 to -30 °C, with ultralow voltage polarizations at various current densities and ultralong lifespan of >4000 h. The outstanding electrochemical properties enlist full cell of zinc-ion batteries constructed with nanoporous Cu/Al2Cu and ZnxV2O5/C to maintain high capacity and excellent stability for >5000 cycles at 25 and -30 °C.

Fundamental Understanding of Hydrogen Evolution Reaction on Zinc Anode Surface: A First-Principles Study

Hydrogen evolution reaction (HER) has become a key factor affecting the cycling stability of aqueous Zn-ion batteries, while the corresponding fundamental issues involving HER are still unclear. Herein, the reaction mechanisms of HER on various crystalline surfaces have been investigated by first-principle calculations based on density functional theory. It is found that the Volmer step is the rate-limiting step of HER on the Zn (002) and (100) surfaces, while, the reaction rates of HER on the Zn (101), (102) and (103) surfaces are determined by the Tafel step. Moreover, the correlation between HER activity and the generalized coordination number ([Formula: see text]) of Zn at the surfaces has been revealed. The relatively weaker HER activity on Zn (002) surface can be attributed to the higher [Formula: see text] of surface Zn atom. The atomically uneven Zn (002) surface shows significantly higher HER activity than the flat Zn (002) surface as the [Formula: see text] of the surface Zn atom is lowered. The [Formula: see text] of surface Zn atom is proposed as a key descriptor of HER activity. Tuning the [Formula: see text] of surface Zn atom would be a vital strategy to inhibit HER on the Zn anode surface based on the presented theoretical studies. Furthermore, this work provides a theoretical basis for the in-depth understanding of HER on the Zn surface.

Asymmetric Electrolytes Design for Aqueous Multivalent Metal Ion Batteries

With the rapid development of portable electronics and electric road vehicles, high-energy-density batteries have been becoming front-burner issues. Traditionally, homogeneous electrolyte cannot simultaneously meet diametrically opposed demands of high-potential cathode and low-potential anode, which are essential for high-voltage batteries. Meanwhile, homogeneous electrolyte is difficult to achieve bi- or multi-functions to meet different requirements of electrodes. In comparison, the asymmetric electrolyte with bi- or multi-layer disparate components can satisfy distinct requirements by playing different roles of each electrolyte layer and meanwhile compensates weakness of individual electrolyte. Consequently, the asymmetric electrolyte can not only suppress by-product sedimentation and continuous electrolyte decomposition at the anode while preserving active substances at the cathode for high-voltage batteries with long cyclic lifespan. In this review, we comprehensively divide asymmetric electrolytes into three categories: decoupled liquid-state electrolytes, bi-phase solid/liquid electrolytes and decoupled asymmetric solid-state electrolytes. The design principles, reaction mechanism and mutual compatibility are also studied, respectively. Finally, we provide a comprehensive vision for the simplification of structure to reduce costs and increase device energy density, and the optimization of solvation structure at anolyte/catholyte interface to realize fast ion transport kinetics.

Solubilization of Fully Hydrolyzed Poly(vinyl alcohol) at Room Temperature for Fabricating Recyclable Hydrogels

The versatility of poly(vinyl alcohol) (PVA) makes it extensively utilized across various industries, while the solubilization of PVA in aqueous media is essential for its applications. However, the high crystallinity of the fully hydrolyzed PVA poses a big challenge in terms of its dissolution in aqueous media at room temperature. In this work, we present a straightforward, efficient, and safe strategy to achieve this objective by the integration of inorganic additives. The crucial aspect of additives lies in the interference of hydrogen bonds and breaking of the crystal domain within PVA chains, therefore greatly enhancing the solubility. At the optimal condition, the solubility of PVA can reach up to 45 wt% at 25 °C in 4 M HBr solution. It is further proven that the solubility of PVA follows the Hofmeister series well, where the chaotropes facilitate the solubilization process. In addition, the solubility is also significantly determined by the PVA type and additive concentration. By harnessing this feature, we successfully engineer recyclable PVA hydrogels with programmable mechanical properties. The hydrogels exhibit remarkable recyclability by affording a minimum of 8 regeneration cycles without experiencing significant deterioration in mechanical properties. Collectively, this research may significantly contribute to the advancement of PVA applications.

Coupling of Adhesion and Anti-Freezing Properties in Hydrogel Electrolytes for Low-Temperature Aqueous-Based Hybrid Capacitors

Solid-state zinc-ion capacitors are emerging as promising candidates for large-scale energy storage owing to improved safety, mechanical and thermal stability and easy-to-direct stacking. Hydrogel electrolytes are appealing solid-state electrolytes because of eco-friendliness, high conductivity and intrinsic flexibility. However, the electrolyte/electrode interfacial contact and anti-freezing properties of current hydrogel electrolytes are still challenging for practical applications of zinc-ion capacitors. Here, we report a class of hydrogel electrolytes that couple high interfacial adhesion and anti-freezing performance. The synergy of tough hydrogel matrix and chemical anchorage enables a well-adhered interface between hydrogel electrolyte and electrode. Meanwhile, the cooperative solvation of ZnCl2 and LiCl hybrid salts renders the hydrogel electrolyte high ionic conductivity and mechanical elasticity simultaneously at low temperatures. More significantly, the Zn||carbon nanotubes hybrid capacitor based on this hydrogel electrolyte exhibits low-temperature capacitive performance, delivering high-energy density of 39 Wh kg-1 at -60 °C with capacity retention of 98.7% over 10,000 cycles. With the benefits of the well-adhered electrolyte/electrode interface and the anti-freezing hydrogel electrolyte, the Zn/Li hybrid capacitor is able to accommodate dynamic deformations and function well under 1000 tension cycles even at -60 °C. This work provides a powerful strategy for enabling stable operation of low-temperature zinc-ion capacitors.

Strategies for Optimizing the Zn Anode/Electrolyte Interfaces Toward Stable Zn-Based Batteries

Aqueous rechargeable Zn-ion batteries (ARZIBs) have attracted extensive attention because of the advantages of high energy density, high safety, and low cost. However, the commercialization of ARZIBs is still challenging, mainly because of the low efficiency of Zn anodes. Several undesirable reactions (e.g., Zn dendrite and byproduct formation) always occur at the Zn anode/electrolyte interfaces, resulting in low Coulombic efficiency and rapid decay of ARZIBs. Motivated by the great interest in addressing these issues, various optimization strategies and related mechanisms have been proposed to stabilize the Zn anode-electrolyte interfaces and enlengthen the cycling lifespan of ARZIBs. Therefore, considering the rapid development of this field, updating the optimization strategies in a timely manner and understanding their protection mechanisms are highly necessary. This review provides a brief overview of the Zn anode/electrolyte interfaces from the fundamentals and challenges of Zn anode chemistry to related optimization strategies and perspectives. Specifically, these strategies are systematically summarized and classified, while several representative works are presented to illustrate the effect and corresponding mechanism in detail. Finally, future challenges and research directions for the Zn anode/electrolyte interfaces are comprehensively clarified, providing guidelines for accurate evaluation of the interfaces and further fostering the development of ARZIBs.

Ultrasensitive Flexible Thermal Sensor Arrays based on High-Thermopower Ionic Thermoelectric Hydrogel

Ionic circuits using ions as charge carriers have demonstrated great potential for flexible and bioinspired electronics. The emerging ionic thermoelectric (iTE) materials can generate a potential difference by virtue of selective thermal diffusion of ions, which provide a new route for thermal sensing with the merits of high flexibility, low cost, and high thermopower. Here, ultrasensitive flexible thermal sensor arrays based on an iTE hydrogel consisting of polyquaternium-10 (PQ-10), a cellulose derivative, as the polymer matrix and sodium hydroxide (NaOH) as the ion source are reported. The developed PQ-10/NaOH iTE hydrogel achieves a thermopower of 24.17 mV K-1 , which is among the highest values reported for biopolymer-based iTE materials. The high p-type thermopower can be attributed to thermodiffusion of Na+ ions under a temperature gradient, while the movement of OH- ions is impeded by the strong electrostatic interaction with the positively charged quaternary amine groups of PQ-10. Flexible thermal sensor arrays are developed through patterning the PQ-10/NaOH iTE hydrogel on flexible printed circuit boards, which can perceive spatial thermal signals with high sensitivity. A smart glove integrated with multiple thermal sensor arrays is further demonstrated, which endows a prosthetic hand with thermal sensation for human-machine interaction.

Enhancing Hydrophilicity of Thick Electrodes for High Energy Density Aqueous Batteries

Thick electrodes can substantially enhance the overall energy density of batteries. However, insufficient wettability of aqueous electrolytes toward electrodes with conventional hydrophobic binders severely limits utilization of active materials with increasing the thickness of electrodes for aqueous batteries, resulting in battery performance deterioration with a reduced capacity. Here, we demonstrate that controlling the hydrophilicity of the thicker electrodes is critical to enhancing the overall energy density of batteries. Hydrophilic binders are synthesized via a simple sulfonation process of conventional polyvinylidene fluoride binders, considering physicochemical properties such as mechanical properties and adhesion. The introduction of abundant sulfonate groups of binders (i) allows fast and sufficient electrolyte wetting, and (ii) improves ionic conduction in thick electrodes, enabling a significant increase in reversible capacities under various current densities. Further, the sulfonated binder effectively inhibits the dissolution of cathode materials in reactive aqueous electrolytes. Overall, our findings significantly enhance the energy density and contribute to the development of practical zinc-ion batteries.

Electrochromic-Induced Rechargeable Aqueous Batteries: An Integrated Multifunctional System for Cross-Domain Applications

Multifunctional electrochromic-induced rechargeable aqueous batteries (MERABs) integrate electrochromism and aqueous ion batteries into one platform, which is able to deliver the conversion and storage of photo-thermal-electrochemical sources. Aqueous ion batteries compensate for the drawbacks of slow kinetic reactions and unsatisfied storage capacities of electrochromic devices. On the other hand, electrochromic technology can enable dynamically regulation of solar light and heat radiation. However, MERABs still face several technical issues, including a trade-off between electrochromic and electrochemical performance, low conversion efficiency and poor service life. In this connection, novel device configuration and electrode materials, and an optimized compatibility need to be considered for multidisciplinary applications. In this review, the unique advantages, key challenges and advanced applications are elucidated in a timely and comprehensive manner. Firstly, the prerequisites for effective integration of the working mechanism and device configuration, as well as the choice of electrode materials are examined. Secondly, the latest advances in the applications of MERABs are discussed, including wearable, self-powered, integrated systems and multisystem conversion. Finally, perspectives on the current challenges and future development are outlined, highlighting the giant leap required from laboratory prototypes to large-scale production and eventual commercialization.

A Self-Healing PVA-Linked Phytic Acid Hydrogel-Based Electrolyte for High-Performance Flexible Supercapacitors

Flexible supercapacitors can be ideal flexible power sources for wearable electronics due to their ultra-high power density and high cycle life. In daily applications, wearable devices will inevitably cause damage or short circuit during bending, stretching, and compression. Therefore, it is necessary to develop proper energy storage devices to meet the requirements of various wearable electronic devices. Herein, Poly(vinyl alcohol) linked various content of phytic acid (PVA-PAx) hydrogels are synthesized with high transparency and high toughness by a one-step freeze-thaw method. The effects of different raw material ratios and agents on the ionic conductivity and mechanical properties of the hydrogel electrolyte are investigated. The PVA-PA_21% with 2 M H₂SO₄ solution (PVA-PA_21%-2 M H₂SO₄) shows a high ionic conductivity of 62.75 mS cm^-1. Based on this, flexible supercapacitors fabricated with PVA-PA_21%-2 M H₂SO₄ hydrogel present a high specific capacitance at 1 A g^-1 after bending at 90° (64.8 F g^-1) and for 30 times (67.3 F g^-1), respectively. Moreover, the device shows energy densities of 13.5 Wh kg^-1 and 14.0 Wh kg^-1 at a power density of 300 W kg^-1 after bending at 90° and for 30 times during 10,000 cycles. It provides inspiration for the design and development of electrolytes for related energy electrochemical devices.

Simultaneously Stabilizing Both Electrodes and Electrolytes by a Self-Separating Organometallics Interface for High-Performance Zinc-Ion Batteries at Wide Temperatures

Rechargeable aqueous zinc-ion batteries are of great potential as one of the next-generation energy-storage devices due to their low cost and high safety. However, the development of long-term stable electrodes and electrolytes still suffers from great challenges. Herein, a self-separation strategy is developed for an interface layer design to optimize both electrodes and electrolytes simultaneously. Specifically, the coating with an organometallics (sodium tricyanomethanide) evolves into an electrically responsive shield layer composed of nitrogen, carbon-enriched polymer network, and sodium ions, which not only modulates the zinc-ion migration pathways to inhibit interface side reactions but also adsorbs onto Zn perturbations to induce planar zinc deposition. Additionally, the separated ions from the coating can diffuse to the electrolyte to affect the Zn²⁺ solvation structure and maintain the cathode structural stability by forming a stable cathode-electrolyte interface and sodium ions' equilibrium, confirmed by in situ spectroscopy and electrochemical analysis. Due to these unique advantages, the symmetric zinc batteries exhibit an extralong cycling lifespan of 3000 h and rate performance at 20 mA cm^-2 at wide temperatures. The efficiency of the self-separation strategy is further demonstrated in practical full batteries with an ultralong lifespan over 10 000 cycles from -35 to 60 °C.